Japan's growing labor shortage threatens the nation's ubiquitous convenience stores, whose business model relies on an army of part-timers packing bento boxed lunches, manning cash registers and delivering goods 24/7.

The big three operators 7-Eleven, FamilyMart and Lawson, which have expanded through Japan's long slump, are scrambling to ease the pressure on franchisees by offering a mix of financial aid and labor-saving automation.

But their earnings outlook is the bleakest in years.